land degradation & development
Land Degrad. Develop. (2011)
Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.1146
EXCLOSURE LAND MANAGEMENT FOR RESTORATION OF THE SOILS IN
DEGRADED COMMUNAL GRAZING LANDS IN NORTHERN ETHIOPIA
W. MEKURIA1* AND E. AYNEKULU2,3
1
International Water Management Institute (IWMI), PO Box 4199, Ban Nongviengkham, Xaythany District, Vientiane, Laos
2
World Agroforestry Centre (ICRAF), United Nations Avenue, PO Box 30677-00100, Nairobi, Kenya
3
College of Dryland Agriculture and Natural Resources, Mekelle University, PO Box 231, Mekelle, Ethiopia
Received: 13 April 2011; Revised: 21 July 2011; Accepted: 25 July 2011
ABSTRACT
In the northern highlands of Ethiopia, establishment of exclosures to restore degraded communal grazing lands has been practiced for the past
three decades. However, empirical data on the effectiveness of exclosures in restoring degraded soils are lacking. We investigated the influence of exclosure age on degree of restoration of degraded soil and identified easily measurable biophysical and management-related factors
that can be used to predict soil nutrient restoration. We selected replicated (n = 3) 5-, 10-, 15-, and 20-year-old exclosures and paired each
exclosure with samples from adjacent communal grazing lands. All exclosures showed higher total soil nitrogen (N), available phosphorus
(P), and cation exchange capacity than the communal grazing lands. The differences varied between 2!4 ("0!61) and 6!9 ("1!85) Mg ha#1
for the total N stock and from 17 ("3) to 39 ("7) kg ha#1 for the available P stock. The differences in N and P increased with exclosure age.
In exclosures, much of the variability in soil N (R2 = 0!64) and P (R2 = 0!71) stocks were explained by a combination of annual average
precipitation, woody biomass, and exclosure age. Precipitation and vegetation canopy cover also explained much of the variability in soil
N (R2 = 0!74) and P (R2 = 0!52) stocks in communal grazing lands. Converting degraded communal grazing lands into exclosures is a viable
option to restore degraded soils. Our results also confirm that the possibility to predict the changes in soil nutrient content after exclosure
establishment using regression models is based on field measurements. Copyright © 2011 John Wiley & Sons, Ltd.
keywords:
exclosures; field measurements; native vegetation; restoration of degraded lands; soil properties; Ethiopia
INTRODUCTION
Natural resources are the primary source of livelihood
support for the poor, and efforts to rehabilitate them could
become a valuable strategy for livelihood improvement
(Shylendra, 2002). However, in many developing countries,
the resource base has been deteriorating over time. In the
highlands of Ethiopia, deforestation and subsequent unsustainable agricultural management, as well as use of dung
and crop residues for energy, have resulted in soil organic
matter and nutrient depletion, hydrological instability,
reduced primary productivity, and low biological diversity (Solomon et al., 2002; Mulugeta et al., 2005). For
example, Brhane and Mekonen (2009) reported soil loss of
35 t ha#1 y#1 from cultivated steep slopes (30–50%) in the
Northern Highlands of Ethiopia, which is about twice the
maximum tolerable soil loss of the region. Furthermore,
owing to overgrazing, the natural vegetation in the Northern
*Correspondence to: W. Mekuria, International Water Management Institute
(IWMI), PO Box 4199, Ban Nongviengkham, Xaythany District, Vientiane,
Laos.
E-mail: wolde_mekuria@yahoo.com
Copyright © 2011 John Wiley & Sons, Ltd.
Highlands of Ethiopia has virtually disappeared, leaving degraded communal grazing lands with irregularly spaced
trees and shrubs and vast areas of bare lands devoid of
vegetation (Nedessa et al., 2005).
In response to the environmental problems, communities
in the Northern Highlands of Ethiopia started to establish
exclosures about three decades ago. Exclosures are areas
closed off from the interference of human and domestic animals with the goal of promoting natural regeneration of
plants and reducing land degradation of formerly degraded
communal grazing lands. Exclosures are usually established
in steep, eroded, and degraded areas that have been used
for grazing in the past (Descheemaeker et al., 2006). In
Ethiopia, the inception of exclosures dates back to the
1980s and coincided with the introduction of large-scale
land rehabilitation and soil and water conservation programmes (Nedessa et al., 2005). Priority areas for establishing exclosures are normally identified as a joint initiative of
local communities and governmental and nongovernmental
organizations (Descheemaeker et al., 2006). As the exclosures are not fenced, guards are hired by the local administration on a food-for-work basis (Yayneshet et al., 2009).
According to the local agricultural offices, exclosures now
W. MEKURIA AND E. AYNEKULU
cover 3–16% of the total area in the study districts of Atsbi
Womberta, Douga Tembein, Axum, and Enda Mehoni.
Grazing impacts on soil properties have generally been
shown to be dependent on grazing intensity. With moderate
grazing of 33 years compared with an ungrazed control,
higher values were found for pH, available P, and Mg.
Concentrations of available P, total N, Ca, Mg, and K
decreased after 1!5 years of heavy grazing compared with
an ungrazed control in a tropical pasture (Ajorlo et al.,
2011). In addition, heavy grazing resulted in lower water infiltration (Hiernaux et al., 1999) and higher soil loss
(Tadesse and Penden, 2002) compared with moderately
grazed sites. Indeed, in semi-arid ecosystems, areas used
by domestic livestock have shown less plant cover and less
soil organic carbon and N compared with relict sites
browsed by native ungulates (Fernandez et al., 2008). In
Tunisia, Jeddi and Chaieb (2010) documented that 12-year
exclosures enhance the total plant cover, dry matter yield,
species richness, contents of soil organic matter, total
nitrogen, and water infiltration rate compared with continually grazed area. Similarly, Cheng et al. (2011) indicated
that 20-year exclusion of livestock grazing significantly
increased aboveground and belowground biomass and
species richness for five different communities compared
with that before exclusion of livestock grazing in a typical
steppe of the Loess plateau, northwest China.
Similar trends were also reported from case studies conducted on exclosures in the central and northern highlands
of Ethiopia: exclosures had twice the plant species richness
and diversity value compared with communal grazing lands
after 22 years of exclosure establishment (Tefera et al.,
2005), an increase in woody species richness of 13 after
8 years of exclosure establishment (Emiru et al., 2006),
and an increase in soil organic matter of 1!1%, total N
of 0!1%, and available P of 1!8 mg kg#1 after 10 years of
exclosure establishment (Mekuria et al., 2007). Also, a
considerable decrease in soil loss was reported after the
establishment of exclosures on communal grazing lands
(Descheemaeker et al., 2006; Girmay et al., 2009; Mekuria
et al., 2009). The study by Mekuria et al. (2007) is limited
in one district, only covered two exclosure ages, and did not investigate the edaphic and environmental factors that influence
the effectiveness of exclosures to restore degraded soils. A replicated study across multiple sites is needed to investigate the
relationship between soil nutrient stocks, soil properties, native
vegetation composition, exclosure duration, and environmental factors affecting restoration of degraded soils. Such information is crucial for planning further establishment of exclosures.
We carried out the present study in the northern highlands
of Ethiopia with the following objectives: (i) to investigate
how exclosure age affects restoration of soil nutrient stocks
and properties and (ii) to identify which easily measurable
biophysical and management-related factors, such as
Copyright © 2011 John Wiley & Sons, Ltd.
exclosure age, can be used to predict restoration of soil nutrient stocks. Easily measurable variables that we considered in
this study are variables related to climate (e.g., precipitation),
soil (e.g., texture), and vegetation (e.g., vegetation cover and
woody biomass). Based on a review of existing case studies,
we formulated the following hypotheses: (i) soil nutrient
content and properties will be improved after establishing
exclosures on communal grazing lands, and (ii) a considerable proportion of the variability in soil nutrient stocks in
exclosures and grazing lands can be explained by a combination of edaphic and site variables as well as exclosure age.
The analyzed soil nutrient content from exclosures was compared with those of adjacent communal grazing lands. The
trends in the changes observed were then used as indicators
of exclosure effectiveness to restore degraded soils.
MATERIALS AND METHODS
Study Area and Experimental Design
Our study was conducted in four districts: Atsbi Womberta
(13$ 53′N, 39$ 45′E), Douga Tembein (13$ 37′N, 39$ 13′E),
Axum (14$ 12′N, 38$ 42′E), and Enda Mehoni (13$ 37′N,
39$ 13′E) located in the eastern, southeastern, central, and
southern zone of the Tigray region (12$ –15$ N latitude and
36$ 30′–40$ 30′ E longitude), the northernmost region of
Ethiopia, respectively (Figure 1). These four districts were
selected because they all have an extensive exclosure system
and adjacent communal grazing lands that have comparable
edaphic and topographic features. All the sites have a
tropical, semi-arid climate. The mean annual rainfall (for
2000–2006) varied between 578 and 671 mm y#1, with an
average of 609 mm y#1. The mean minimum temperature
(2000–2006) ranged from 7!8 to 11!6 $ C, and the mean maximum temperature ranged from 22!2 to 28!2 $ C (Ethiopian
Meteorological Service Agency, 2007). The altitude of
the study sites ranged from 2232 to 2937 m above sea
level. The rainy season usually occurs between June and
September, with a growing season of between 90 and
120 days. Soils of the study sites were classified into four
major groups: Luvisols, Regosols, Cambisols, and Calcisols
(WRB, 2006). Selected soil and vegetation characteristics
are given in Table I. The most common woody vegetation species in exclosures and in adjacent grazing lands
included Acacia etbaica Schweinf., Acacia seyal Del.,
Becium grandiflorum (Lam.) Pichi-Serm., Euclea racemosa
ssp. schimperi (A.DC.) Dandy, and Maytenus arbutifolia
(Hochst. ex. A. Rich) Wilczek. Understory vegetation of
the exclosures and the adjacent communal grazing lands
were dominated by grass species such as Hyparrhenia hirta
L. and Digitaria velutina (Forssk) P. Beauv.
We selected replicated (n = 3) 5-, 10-, 15-, and 20-year-old
exclosures that were adjacent to corresponding communal
grazing lands. Before establishment, exclosures and the
LAND DEGRADATION & DEVELOPMENT (2011)
EXCLOSURES RESTORE DEGRADED SOIL
Table I. Mean values (n = 3, SE in parentheses) of selected soil
and vegetation characteristics in paired exclosures and adjacent
grazing lands
Variables
Plant species
richness (number)
Diversity
(Shannon–
Wiener index)
Aboveground
standing
biomass (Mg ha#1)
Canopy cover (%)
Under-canopy
cover (%)
Area of the total
sampled plots (ha)
Sand (%)
Figure 1. Location of the study area in the highlands of Tigray, Northern
Ethiopia, with the distribution of specific sites indicated by (▪). This figure
is available in colour online at wileyonlinelibrary.com/journal/ldr
adjacent grazing lands had similar conditions because the
exclosures were established on parts of the degraded communal grazing lands that were used for livestock grazing
(Mekuria et al., 2011). In exclosures, grass harvesting (using
a cut-and-carry system) is allowed and is conducted once a
year, typically after seeding stage, starting 3 to 5 years after
exclosure establishment. The main reason to restrict grass
harvesting is to restore the soil seed bank. On the adjacent
communal grazing lands, unlimited access for free grazing
is practiced. The stocking rate of the communal grazing
lands, determined using the average animal weight method
(Pratt and Rasmussen, 2001), varied between 97 and 201
animals per month. The area of the exclosures ranged from
7 to 28 ha, whereas the adjacent communal grazing lands
ranged from 2 to 12 ha. Accordingly, the total available
forage that can be obtained from the grazing lands can only
support 8!1 to 16!8 animals for a 12-month period (Mekuria
et al., 2011). Given that a minimum of 80 livestock (mainly
cows and oxen) graze on the communal grazing lands
throughout the year, the communal grazing lands are under
heavy grazing pressure resulting in severe land degradation.
Copyright © 2011 John Wiley & Sons, Ltd.
Silt (%)
Clay (%)
Bulk density (g cm#3)
pH (1:5 H2O)
Exclosure
age (year)
Exclosures
Adjacent
grazing
lands
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
20!0 (1!7)*
15!7 (1!8)
30!0 (6!8)*
31!3 (4!2)*
1!4 (0!1)
1!4 (0!1)*
2!0 (0!1)*
2!1 (0!1)*
4!2 (0!6)*
9!9 (1!7)*
13!6 (1!8)*
15!0 (2!8)*
28!4 (3!4)*
28!9 (3!3)*
34!0 (3!6)*
43!1 (4!1)*
56!8 (4!1)*
46!7 (5!8)*
60!9 (3!7)*
54!4 (6!3)*
0!96
1!08
0!96
1!08
58!1 (2!8)
53!9 (4!3)
60!4 (3!4)
48!9 (4!1)
29!1 (1!9)
26!4 (2!5)
23!8 (1!9)
30!0 (3!1)
12!8 (1!1)
19!7 (2!6)
15!8 (2!8)
21!1 (3!8)
1!3 (0!1)
1!3 (0!1)
1!1 (0!1)
1!1 (0!0)
6!2 (0!1)
5!9 (0!1)
6!2 (0!1)
6!0 (0!1)
10!0 (0!3)*
6!7 (4!1)
10!7 (2!7)*
11!7 (4!8)*
1!3 (0!1)
0!7 (0!2)*
0!9 (0!2)*
1!4 (0!2)*
0!7 (0!2)*
1!1 (0!4)*
0!1 (0!0)*
2!0 (0!6)*
14!1 (2!6)*
9!4 (3!4)*
10!4 (3!5)*
15!1 (2!7)*
17!6 (3!6)*
20!2 (5!8)*
23!9 (2!3)*
21!5 (10!9)*
0!96
0!60
0!72
0!96
64!6 (1!9)
57!9 (4!7)
55!1 (2!7)
49!3 (3!2)
24!4 (2!1)
27!4 (3!9)
28!0 (2!6)
27!3 (1!7)
11!0 (0!9)
14!7 (0!9)
16!9 (3!4)
23!3 (4!0)
1!3 (0!1)
1!3 (0!1)
1!2 (0!1)
1!3 (0!0)
6!3 (0!1)
5!8 (0!2)
6!1 (0!1)
6!1 (0!1)
*Differences between exclosures age and the adjacent grazing lands are
significant at P < 0!05 after paired t-test.
We used a space-for-time substitution approach to monitor changes in soil nutrient stock and properties after conversion of communal grazing lands to exclosures with ages of
5, 10, 15, and 20 years. Thus, we selected replicated (n = 3)
5-, 10-, 15-, and 20-year-old exclosures that were adjacent
LAND DEGRADATION & DEVELOPMENT (2011)
W. MEKURIA AND E. AYNEKULU
to a corresponding communal grazing lands, ensuring that
soil and terrain conditions were as similar as possible
between each pair. We also selected three isolated forest
fragments that remain around churches, also called “church
forests,” to detect whether exclosures had reached soil nutrient stock levels of these forests. In each exclosure, grazing
land and church forest, we randomly established two to three
transects spaced at a minimum distance of 75 m. The number of transects per site was based on vegetation density,
spatial heterogeneity of vegetation, and area of the site
(Tefera et al., 2005). To avoid edge effects, the first transect
was laid 30 to 50 m inside the exclosures, grazing land, or
church forest. Transects were parallel to each other and to
the topography of the landscape. In each transect, we delineated three landscape positions (upper slope, mid-slope, and
foot slope), and in each landscape position, we established a
sampling plot of 20 m % 20 m for quantitative vegetation
inventory. In each 20 m % 20 m plot, we measured the
following individual plant variables: diameter at breast
height or, for smaller and multi-stemmed shrub, diameter
at stump height or at the height of 30 cm (d30) from the
ground, crown diameter, and total height. We also identified
the species identity of plants encountered in each plot. During the entire study, we examined 210 plots, of which 105
were in exclosures, 81 in communal grazing lands, and 24
in church forests. In each 20 m % 20 m plot, five 2 m % 2 m
subplots were set up for soil sampling (one at the center
and two along each of the two diagonal lines that crosses
at the center of the plot). Soil, vegetation, and managementrelated data were collected from November 2007 to
October 2008.
Soil Sampling, Analyses, Soil Nutrient Stocks Calculation,
and Collection of Site Variables
In each 2 m % 2 m subplot, soil samples at 0- to 0!2-m depth
were collected at five random sampling points. One soil core
sample was also taken from the central plot of each
landscape position for bulk density determination (i.e., we
collected 81 soil core samples during the entire study). The
samples collected from each landscape position per replicate
(i.e., 50–75 random sampling points from two to three
transects) were mixed thoroughly in a large bucket to form
one composite soil sample. During the entire study, we
collected a total number of 81 composite soil samples. Soil
samples were air-dried, sieved through a 2-mm sieve, and
ground before analysis. Total soil N (N) was analyzed using
the Kjeldahl method (Bremmer and Mulvaney, 1982), available P was analyzed using the Olsen-P method (Olsen and
Sommers, 1982), bulk density was measured using the core
method (Blake and Hartge, 1986), particle size was determined using the hydrometer method (Gee and Bauder,
1982), and pH was determined using a suspension of 1:5
soil : water ratio. Ammonium and sodium acetate extracts
Copyright © 2011 John Wiley & Sons, Ltd.
were used to determine exchangeable cations and cation exchange capacity (CEC), respectively (Thomas, 1982).
Total soil N and available P stocks in the 0- to 0!2-m layer
were calculated as follows:
!
" !
"
N Mg N ha#1 ¼ N ½%( % 10#2 % BdðMg m#3 Þ
% depthðmÞ % 10000 m2 ha#1
and
!
" !
"
Olsen-P Mg Pha#1 ¼ PðppmÞ % 10#6 % BdðMg m#3 Þ
% depthðmÞ % 10000 m2 ha#1
Where: N, Olsen-P, and Bd are the total soil N, available P,
and bulk density, respectively. The average bulk density of
the oldest exclosure was used to calculate the total soil N
and available P stocks in all exclosures, grazing lands, and
church forests. We used this conservative approach to avoid
overestimation of the total soil N and available P stocks due
to changes in bulk density (Veldkamp, 1994).
We used the methods of Hoff et al. (2002) and Snowdon
et al. (2002) for aboveground woody biomass measurement.
We harvested altogether 423 trees and shrubs (276 from
exclosures and 147 from communal grazing lands). In the
church forests, the partial harvest method was used because
tree felling is strictly forbidden; biomass was collected from
60 individuals. Fresh mass of the aboveground vegetation
was adjusted to the dry mass using the measured moisture
contents, determined by oven-drying subsamples of stems,
branches, and leaves at 65 $ C until constant mass was
attained (about 78 h). Vegetation canopy cover was determined using crown diameter measurements and by assuming a tree or shrub of elliptical shape (Snowdon et al.,
2002). The average annual rainfall for the years 2000 to
2006 of the study sites was obtained from Ethiopian Meteorological Service Agency. The altitude and slope of the
study sites were measured at each landscape position using
an altimeter and clinometer, respectively. Stone cover was
quantified using the line–point intercept method (Herrick
et al., 2005).
Statistical Analyses
We first conducted tests for normality (Kolmogorov–
Smirnov D statistic) and equality of variance (Levene statistic) of the total soil N and available P concentrations, total
soil N and P stocks, and CEC. We also tested the differences
among the landscape positions in the soil nutrient content
and properties for each exclosure age and communal grazing
land using one-way analysis of variance (ANOVA). The differences in soil nutrient content and soil properties between
an exclosure age and adjacent communal grazing land were
assessed at each landscape position using a paired t-test with
separate variances because the tested parameters did not
show homogenous variances. The patterns of changes in
the total soil N and available P stocks (i.e., difference
LAND DEGRADATION & DEVELOPMENT (2011)
EXCLOSURES RESTORE DEGRADED SOIL
between an exclosure age and adjacent grazing lands) across
exclosure ages (5, 10, 15 and 20 years) were assessed using
Kruskal–Wallis test (nonparametric ANOVA) with nonparametric Tukey’s test. Spearman correlation tests were conducted to examine the relationships between soil nutrient
content and properties and independent variables using the
mean values of the three landscape position for each land
use type (n = 12). Finally, stepwise multiple regression analysis was done to establish predictive relationships between
soil N and P stocks and easily measurable independent
variables: precipitation, woody biomass, vegetation canopy
cover, under-canopy cover, exclosure age, percentage clay,
percentage silt, percentage sand, soil pH, bulk density, and
stone cover. We selected the best predictive variables using
a forward stepwise procedure that uses a sequence of F tests
at P < 0!05. We did not detect any colinearity among the input predictive variables used in the regression models.
RESULTS
Soil Nutrient Content and Soil Properties in Exclosures and
Adjacent Grazing Lands
We did not detect differences among landscape positions
(P > 0!05) in soil nutrient content and properties within
any of the exclosure ages and adjacent grazing lands, suggesting that landscape position was not a significant conditioning factor affecting soil nutrient content and soil
properties. At each landscape position, exclosures showed
significantly higher total soil N content, available P content,
and CEC than the adjacent grazing lands (Table II).
The difference in soil properties between exclosures and
adjacent grazing lands in the 0- to 0!2-m depth, averaged
across landscape positions at each exclosure age, varied between 2!4 ("0!61) and 6!9 ("1!85) Mg ha#1 for the total N
stocks, from 17 ("3) to 39 ("7) kg ha#1 for the available
P stocks, and between 13!0 ("1!2) and 20!4 ("2!8) for
CEC. These values translated to an increase of 28–48% for
the total soil N stocks, 26–39% for the available P stocks,
and 47–71% for the CEC after conversion of the degraded
grazing lands to exclosures. The influence of exclosure age
on the total soil N, available P stocks, and CEC was not
linear with time because the increase in the total soil N
and available P stocks was only significant between 5- and
20-year-old exclosures, and we observed marginally
significant differences (P = 0!058) in CEC between 5- and
20-year-old exclosures (Figure 2a–c).
Soil, Vegetation, and Climatic Variables Explaining Soil
Nutrient Content and Soil Properties
The total soil N and available P concentrations and stocks as
well CEC in exclosures and their differences from adjacent
grazing lands were positively correlated with woody
Copyright © 2011 John Wiley & Sons, Ltd.
biomass, vegetation canopy cover, clay content, precipitation, and exclosure age but were inversely correlated with
bulk density (Table III). In grazing lands, the total soil N
concentration and stock did not show significant positive
and significant negative correlations with the independent
variables. Available P stock and CEC were positively correlated only with vegetation canopy cover.
Using multiple linear regression models, we were able to
explain much of the variability in the total soil N and available P stored in exclosures and communal grazing lands
(Table IV). The total soil N stock in exclosures was predicted
by woody biomass (ranging from 3!3 to 17!5 Mg ha#1), precipitation (ranging from 578 to 671 mm y#1), and exclosure
age (ranging from 5 to 20 years), whereas the available P
stock was predicted by exclosure age and woody biomass.
In grazing lands, the total soil N stock was predicted only
by annual average precipitation (ranging from 578 to
671 mm y#1), whereas available P stock was predicted by
precipitation and vegetation canopy cover (ranging from
5% to 18%). The differences between exclosures and grazing
lands in the total soil N and available P stocks were predicted
by clay content (ranging from 11!7% to 26%) and woody biomass (ranging from 3!3 to 17!5 Mg ha#1).
Soil Nutrient Content and Soil Properties in Church Forests
Soils of the church forest had 0!68% ("0!13%) total soil N,
40!4 ("1!37) mg kg#1 available P, and 43!8 ("1!81) cmolc
kg#1 CEC. The total soil N and available P stocks were
15!7 ("3!29) Mg ha#1 and 0!09 ("0!0) Mg ha#1, respectively. The 20-year-old exclosure had reached the total soil
N and available P levels (Table II) comparable with the
church forests.
DISCUSSION
Variation of Soil Nutrient Content on Communal Grazing
Lands
The studied communal grazing lands displayed different
degrees of land degradation, which is illustrated by the variation in vegetation composition, aboveground biomass, and
soil nutrient content and properties (Tables I and II). The
variation in soil degradation among the studied communal
grazing lands could arise from the difference in the interference of human and domestic grazing animals, which consequently resulted in different degrees of vegetation
degradation (Table I). Other studies also indicated that anthropogenic disturbance and livestock grazing have a strong
negative effect on species composition, germination, seedling growth, and mortality of many of the plant communities
and in turn results in less species richness and poor soil quality in severely grazed sites (Singh and Singh, 1987; Wassie
et al., 2009).
LAND DEGRADATION & DEVELOPMENT (2011)
Copyright © 2011 John Wiley & Sons, Ltd.
Foot
Mid
Upper
Foot
Mid
Upper
Foot
Mid
Upper
Foot
Mid
Upper
Landscape
position
0!34
(0!0)
0!38
(0!0)
0!39
(0!1)
0!45
(0!1)
0!43
(0!1)
0!41
(0!1)
0!41
(0!0)
0!57
(0!2)
0!54
(0!1)
0!52
(0!0)
0!50
(0!2)
0!85
(0!0)
N
(%)
34!6
(2!1)
32!8
(3!6)
35!5
(2!5)
34!2
(0!9)
34!1
(6!7)
35!7
(0!7)
35!2
(0!7)
39!3
(1!9)
49!5
(9!3)
41!7
(1!5)
37!9
(2!6)
45!9
(10!2)
9!9 (0!4)
38!9 (0!7)
52!8 (6!5) 19!0 (0!3)
46!2 (2!9) 11!1 (3!2)
48!0 (0!2) 12!6 (0!6)
42!0 (1!7) 12!0 (1!6)
40!9 (1!2) 12!6 (3!2)
9!2 (1!0)
9!5 (0!9)
10!9 (1!1)
8!6 (1!7)
8!4 (0!6)
8!3 (0!2)
40!1 (1!4)
40!0 (0!9)
41!9 (0!9)
41!2 (2!7)
39!6 (0!8)
40!1 (2!2)
0!08
(0!01)
0!07
(0!01)
0!08
(0!01)
0!08
(0!00)
0!08
(0!01)
0!08
(0!00)
0!09
(0!00)
0!09
(0!00)
0!11
(0!02)
0!10
(0!00)
0!08
(0!01)
0!10
(0!02)
0!27
(0!0)
0!24
(0!0)
0!29
(0!0)
0!23
(0!0)
0!25
(0!0)
0!24
(0!0)
0!26
(0!0)
0!32
(0!0)
0!25
(0!0)
0!21
(0!1)
0!41
(0!1)
0!35
(0!1)
27!7
(4!6)
28!7
(3!8)
24!3
(4!0)
22!3
(3!8)
24!4
(2!3)
25!2
(2!5)
25!9
(3!1)
26!6
(2!6)
26!7
(1!6)
23!9
(1!2)
24!6
(1!4)
26!4
(1!2)
5!5 (0!3)
24!3 (0!4)
28!2 (1!3)
28!8 (2!0)
7!9 (2!1)
9!1 (0!5)
5!0 (0!8)
7!1 (0!7)
24!0 (0!7)
28!8 (2!9)
6!3 (0!4)
5!3 (0!7)
5!4 (0!2)
25!5 (1!5)
28!3 (2!8)
24!7 (1!4)
5!5 (0!4)
6!4 (0!7)
28!1 (2!4)
23!6 (1!4)
5!3 (0!6)
6!5 (0!4)
27!3 (3!1)
26!6 (3!3)
0!07
(0!01)
0!06
(0!01)
0!05
(0!01)
0!05
(0!01)
0!05
(0!01)
0!06
(0!01)
0!06
(0!01)
0!06
(0!01)
0!06
(0!00)
0!06
(0!00)
0!05
(0!00)
0!06
(0!00)
N
P
N
P
CEC
N
P
P
CEC
(mg/g) (cm[+]kg#1) (Mg ha#1) (Mg ha#1) (%) (mg/g) (cm[+]kg#1) (Mg ha#1) (Mg ha#1)
Adjacent grazing lands
b
Differences
b
14*
(2!1)
12*
(3!0)
13*
(1!5)
18**
(1!9)
15**
(0!7)
12*
(2!8)
13**
(2!2)
17**
(1!8)
18**
(1!8)
19*
(2!9)
17*
(4!9)
25*
(7!0)
1!8*
(0!6)
3!1*
(0!2)
2!2
(1!9)
5!4**
(1!2)
4!0*
(1!0)
3!9*
(0!4)
3!6**
(0!8)
5!5
(2!5)
6!5*
(1!6)
7!6**
(1!1)
2!0
(3!6)
11!1**
(2!3)
0!02
(0!01)
0!01
(0!00)
0!03
(0!00)
0!03
(0!01)
0!02
(0!01)
0!02*
(0!00)
0!02
(0!01)
0!03*
(0!01)
0!05
(0!02)
0!04**
(0!00)
0!03*
(0!01)
0!04
(0!02)
CEC
P
N
P
(cm
(mg/g) [+]kg#1) (Mg ha#1) (Mg ha#1)
0!08* 6!9
(0!02) (2!5)
0!14* 4!1
(0!01) (0!2)
0!10 11!2
(0!09) (1!7)
0!22** 11!9
(0!05) (3!8)
0!18* 9!7
(0!04) (5!8)
0!17* 10!5*
(0!02) (2!0)
0!15** 9!3
(0!03) (3!2)
0!25 12!7*
(0!11) (4!4)
0!29* 22!8
(0!07) (8!3)
0!31** 17!8**
(0!05) (2!0)
0!09 13!3*
(0!17) (2!7)
0!50* 19!5
(0!10) (9!9)
N
(%)
Duration of time after the conversion of communal grazing lands to exclosure.
Values are calculated as exclosure values # adjacent grazing land value.
*Differences in soil nutrients concentrations and stocks and soil properties were significant at P < 0!05 (paired t-test with separate/unequal variances).
**Differences in soil nutrients concentrations and stocks and soil properties were significant at P < 0!01 (paired t-test with separate/unequal variances).
a
20
15
10
5
Exclosure
age
(years) a
Exclosure
Table II. Mean values ("SE, n = 3) of the total soil N and available P concentration, total soil N and P stocks, and CEC in the 0- to 0!2-m depth in exclosures, adjacent grazing
lands, and their differences
W. MEKURIA AND E. AYNEKULU
LAND DEGRADATION & DEVELOPMENT (2011)
EXCLOSURES RESTORE DEGRADED SOIL
ecosystems is the removal of a major part of the aboveground biomass, consequently the input of the aboveground
litter to the soil decreases, which may have important
consequences for soil nutrient conservation and cycling
(Abule et al., 2005; Savadogo et al., 2007). Furthermore,
livestock grazing could deteriorate hydrological soil properties, mainly structure of the soils, consequently reducing the
rate of microbial process and nutrient retention (Neff et al.,
2005). The poor positive correlation between soil nutrient
and clay content (Table III) could arise from the decrease
in clay content with time in the communal grazing lands
due to soil erosion, which in turn reduced the positive effect
of clay in restoring soil nutrient content.
Our field-based regression models open up possibilities to
predict soil nutrient content in communal grazing lands, which
is important baseline information for the evaluation of exclosures established on degraded communal grazing lands. The
range covered by the independent variables in these models
is typical of the highlands of Tigray because more than 80%
of the highlands of Tigray receives annual rainfall between
500 and 800 mm y#1 (Abay et al., 2008), and the majority
of the communal grazing lands are severely degraded
supporting only sparse vegetation (Nedessa et al., 2005).
Changes in Soil Nutrient Content and Properties in Relation
to Exclosure age
Figure 2. Differences between exclosures and adjacent communal grazing
lands in the total soil N stock (a), available P stock (b), and CEC (c) in relation to exclosure age. Means (n = 3, SE bars) followed by different letters
indicate significant differences among categories of exclosure age (one-way
ANOVA with Kruskal–Wallis test at P ≤ 0!05).
The insignificant and less strong positive correlation of
soil nutrient content in communal grazing lands with woody
species biomass and vegetation canopy cover (Table III)
indicates that free grazing influenced soil nutrient restoration
through reducing organic input to the soil. Other studies also
indicated that a direct impact of grazing on the rangeland
Copyright © 2011 John Wiley & Sons, Ltd.
The inherent assumption of our space-for-time substitution
approach is that the exclosures and adjacent communal grazing lands had similar conditions before exclosure establishment. We tested this assumption using variables measured
in the paired exclosures and grazing lands that are less dependent of land use (Mekuria et al., 2011), and we verified
that the paired sites were comparable, and differences in soil
nutrient content, soil properties, and native vegetation richness and diversity measured between the paired exclosures
and adjacent grazing lands were caused by land use change
and not by inherent site variability.
The higher soil nutrients content and properties in all
exclosures indicate that exclosures have a significant positive effect on the restoration of degraded soils because of
overgrazing (Table II). Similar results were reported from
case studies conducted on exclosures established within
the last two decades in the Central Highlands of Ethiopia:
an increase of 0!67% organic matter, 8!85 mg kg#1 increase
in available P, and 9!18 cmolc kg#1 increase in CEC after
9 years of exclosure establishment (Mamo, 2008) and
2!33%, 0!08%, 7!89 cmolc kg#1 increases in organic matter,
total soil N, and CEC, respectively, after 20 years of exclosure establishment (Tsetargachew, 2008). The considerable
differences in soil nutrient content and properties between
exclosures and communal grazing lands can be explained
either by increased grazing pressure in the reduced areas of
communal grazing lands after establishment of exclosures
LAND DEGRADATION & DEVELOPMENT (2011)
Copyright © 2011 John Wiley & Sons, Ltd.
#0!6
#0!2
0!6*
#0!4
#0!3
0!4
0!3
0!5
0!3
#0!9* #0!6*
#0!4 #0!6*
#0!4
#0!4
0!8*
0!7*
0!6*
#0!6*
0!7*
0!4
#0!6*
0!4
0!2
0!7*
0!1
0!6*
0!7*
0!3*
0!8*
0!7*
0!7*
0!8*
CEC (cm
[+]kg#1)
P
(mg/g)
N
(%)
0!7*
#0!5
#0!3
0!9*
#0!3
0!3
0!5
#0!9*
0!2
0!8*
0!9*
N (Mg
ha#1)
Exclosure
0!8*
#0!5
#0!5
0!7*
#0!1
0!1
0!4
#0!8*
0!5
0!6*
0!8*
P
(Mg ha#1)
#0!1
#0!0
0!3
#0!2
0!2
0!1
#0!1
0!2
0!5
0!0
N
(%)
#0!5
#0!3
0!5
#0!4
0!4
0!4
#0!3
0!1
0!6
0!3
P
(mg/g)
#0!5
#0!0
0!5
#0!5
0!5
0!6
#0!1
0!4
0!8*
0!4
CEC (cm
[+]kg#1)
#0!3
0!3
0!3
#0!4
#0!5
#0!3
0!4
0!0
0!1
0!3
0!0
0!6*
0!5
P (Mg
ha#1)
#0!2
0!2
0!1
#0!1
0!3
0!3
0!0
N
(Mg ha#1)
Adjacent grazing lands
0!1
0!8*
0!8*
P
(mg/g)
0!6*
#0!2
#0!6
0!6*
0!8*
#0!4
#0!4
0!7*
#0!4
#0!5
0!4
0!5
0!7*
0!4
#0!9* #0!8*
0!4
0!6
0!8*
N
(%)
*P < 0!05.
a
Differences are calculated as exclosure values minus adjacent grazing land values and are correlated with site and vegetation characteristics under exclosures.
Woody biomass
(Mg ha#1)
Vegetation canopy cover (%)
Under-canopy
cover (%)
Sand (%)
Silt + clay (%)
Clay (%)
Bulk density
(g cm#3)
Stone cover (%)
Slope (%)
Precipitation
(mm y#1)
Exclosure age
(year)
Site and
vegetation
characteristics
0!7*
#0!5
#0!1
0!7*
#0!4
0!4
0!6*
#0!8*
0!2
0!9*
0!8*
CEC (cm
[+]kg#1)
Differences
a
0!6*
#0!2
#0!5
0!6*
#0!4
0!4
0!7*
#0!9*
0!4
0!5
0!8*
N (Mg
ha#1)
0!8*
#0!5
#0!4
0!7*
#0!5
0!5
0!5
#0!8*
0!1
0!8*
0!8*
P (Mg
ha#1)
Table III. Spearman correlation coefficients (n = 12) between soil nutrients content and properties and site and vegetation characteristics within exclosures and communal grazing
lands
W. MEKURIA AND E. AYNEKULU
LAND DEGRADATION & DEVELOPMENT (2011)
EXCLOSURES RESTORE DEGRADED SOIL
Table IV. Multiple regression models for the prediction of soil nutrient stocks in the 0- to 0!2-m depth in exclosures, communal grazing
lands, and their differences
P-value
R2a
0!01
0!64
0!00
0!74
0!00
0!05
0!71
0!52
Differences between paired exclosures and grazing lands in the total soil nitrogen and available phosphorus stocks
0!00
Total soil N (Mg ha# 1) = # 2.7 + 0.3 % clay (%) + 0.2 % woody biomass (Mg ha-1)
0!00
Soil available P (Mg ha# 1) = 0.002 + 0.002 % woody biomass (Mg ha-1) + 0.0006 % clay (%)
0!79
0!72
Land use group
Model
Total soil nitrogen (N) stock in the 0- to 0!2-m depth
Exclosures
Mg N ha# 1 = # 12.4 + 0.13 % woody biomass (Mg ha-1) + 0.03 % precipitation
(mm y-1) + 0.2 % exclosure age (year)
Grazing lands
Mg N ha# 1 = # 12 + 0.03 % precipitation (mm y-1)
Soil available phosphorus (P) stock in the 0- to 0!2-m depth
Exclosures
Mg P ha# 1 = 0.07 + 0.001 % exclosure age (year) + 0.001 % woody biomass (Mg ha-1)
Grazing lands
Mg P ha# 1 = 0.006 + 0.0001 % precipitation (mm y-1) + 0.001 % vegetation canopy cover (%)
a
We used adjusted R2 values for predictive models with more than two independent variables.
and susceptibility to erosion due to sparse vegetation cover
(Table I) or by increased vegetation cover in the exclosures
(Table I), which would reduce soil erosion and increase organic \matter input into the soil. In contrast to our study,
Tsetargachew (2008) found a higher value of available
phosphorous (9!96 mg kg#1) in communal grazing land
compared with the 20-year-old exclosure (9!33 mg kg#1).
This variation could arise from the less dependence of farmers in Central Highlands of Ethiopia on the use of cow dung
to meet their household energy demand compared with
farmers living in the northern highlands of Ethiopia where
the natural vegetation is entirely disappeared, which resulted
in increased dependence on the use of cow dung and crop
residues for household energy demand (Zenebe, 2007). This
in turn resulted in the removal of all cow dung from the
communal grazing lands and reduction of organic inputs
and soil nutrients to the soil.
Soil nutrient content, mainly the total soil N and available
P restoration, was also influenced by exclosure age. The relatively large increase in the total soil N and available P storage in the first 5 years after establishing exclosures
(Figure 2a and b) may have resulted from the management
rule that restricts grass harvesting for 3 to 5 years after exclosure establishment and subsequent increased organic matter
input derived from herbaceous species biomass, from reduced soil erosion through effective ground cover, and from
relatively slow decomposition under drier and cooler climate
(Mekuria et al., 2011). Furthermore, the 20 years of exclosures had reached the total soil N and available P levels of
church forests, suggesting that a minimum of two decades
is required to increase the soil nutrient content in the communal grazing lands to the level of soil nutrient content in
church forests through exclosure establishment. However,
there could be further potential to restore more total soil N
and available P after about 20 years as the studied church
forests were influenced by human and domestic grazing animals and are not considered as in climax condition.
Copyright © 2011 John Wiley & Sons, Ltd.
The positive correlation of soil nutrient content and soil
properties in exclosures with woody species biomass, vegetation canopy cover, and exclosure duration (Table III) indicate that exclosures influenced soil nutrient content and
soil properties through a higher organic matter input into
the soil with time (Table I). Other studies also reported increasing soil nutrient retention in ecosystem along with the
number of plant species and aboveground biomass (e.g.,
Johannes and David, 2000; Loreau et al., 2001). Furthermore, the increases in canopy cover with the increase in
exclosure duration could decrease sediment-associated soil
nutrient losses by reducing the erosive impact of raindrops
and soil erosion (Tsetargachew, 2008; Girmay et al., 2009;
Mekuria et al., 2009).
Using a combination of climate, soil, vegetation, and
management-related (e.g., exclosure age) variables as independent predictors, we were able to explain a considerable
part of the variation in soil nutrient content after the establishment of exclosures (Table IV). The independent predictor clay content together with precipitation indicates the
importance of water availability to restore degraded soils,
whereas the biomass from woody species indicates the importance of organic inputs. Furthermore, the independent
vegetation predictor percentage canopy cover shows the importance of vegetation cover for reducing soil erosion and
sediment-associated soil nutrient losses and minimizing degradation process. Our regression models based on field data
proved useful for investigating the time dynamics of soil nutrient stocks after the establishment of exclosures, and the
results can provide information for land managers to plan future exclosures as well as to take into account the ecological
importance of such resources in their management decisions.
CONCLUSIONS
The results of the present study confirm that the establishment of exclosures on degraded communal grazing lands
LAND DEGRADATION & DEVELOPMENT (2011)
W. MEKURIA AND E. AYNEKULU
can be effective in improving soil nutrient content and properties. Our study also confirms that it is possible to generate
baseline information and to make quantitative prediction of
the changes in soil nutrient content after the establishment
of exclosures using relatively simple field measurements
and regression models. Such information is critical for evaluating the ecological importance of exclosures and to assist
policy makers to consider the value of exclosures in natural
resource planning and management. Further studies are
needed to investigate the degree to which nutrient enrichment in exclosures is due to soil and nutrient deposition
from surrounding areas, as this may affect the sustainability
of scaling up the area of exclosures.
ACKNOWLEDGEMENTS
The first author acknowledges the financial support by the
Deutscher Akademischer Austauschdients, British Ecological Society, and fieldwork support by Mekelle University
(Ethiopia). The authors thank the anonymous referees for
their constructive comments for improving this paper and
Dr Keith Shepherd for improving the language.
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